The BGS Lexicon of Named Rock Units — Result Details
Cefn Mawr Limestone Formation
| Computer Code: | CFML | Preferred Map Code: | notEntered |
|---|---|---|---|
| Status Code: | Full | ||
| Age range: | Asbian Substage (CR) — Brigantian Substage (CX) | ||
| Lithological Description: | Thinly interbedded dark grey argillaceous limestones (wackestones and packstones) and mudstones with intercalated thick-bedded to massive, pale shelly limestones (packstones and grainstones) and lenticular, cross-bedded, coarsely crinoidal limestone (rudstone) bodies. Lithologies are arranged in cyclic sequences. Limestones at the top of each cycle commonly display calcrete and palaeokarstic features locally overlain by bentonitic clay palaeosols. Thin chert beds and nodules in upper part; rare sandstone beds. The Cefn Mawr Limestone Formation records principally Brigantian platform carbonate deposition on the North Wales Dinantian shelf. Each cyclic sequence records a shoaling upwards unit developed in response to transgressive and regressive movements in sea level. Many regressions culminated in emergence of the platform surface and formation of calcrete and karstic dissolution features. The sequences of thinly interbedded dark grey limestone and mudstone, diagnostic of the Formation, record deposition during marine transgressions, which repeatedly established deeper platformal conditions than ever prevailed during deposition of the preceding and similarly cyclic Loggerheads Limestone Formation. | ||
| Definition of Lower Boundary: | Upper palaeokarstic surface/palaeosol of the Loggerheads Limestone Formation; base of lowest sequence of thinly interbedded, dark grey limestone and mudstones of the Cefn Mawr Limestone Formation. | ||
| Definition of Upper Boundary: | Where overlain by the Minera Formation, the top is taken at the base of the lowest sandstone within the latter; where overlain by the Pentre Chert Formation, the top is taken at the disconformable base of the latter. | ||
| Thickness: | 40 to 275m | ||
| Geographical Limits: | Crops out on Anglesey [SH5080] and the Great Orme [SH7483] (Llandudno) in northwest Wales, in its type area to the east of the Clwydian Range [SJ2063], and at Llangollen [SJ2345] and near Oswestry [SJ2825] in northeast Wales. Also in the Corwen Outlier [SJ0543]. | ||
| Parent Unit: | Clwyd Limestone Group (CLWYD) | ||
| Previous Name(s): | Bishop's Quarry Beds [Obsolete Name and Code: Use CFML]
(-4535)
Bishop's Quarry Limestone Formation [Obsolete Name And Code: Use CFML] (BQB) Trefor Limestone Formation [Obsolete Name And Code: Use CFML] (TLF) Traeth Bychan Limestone Formation [Obsolete Name And Code: Use CFML] (TRL) Grey Limestone, Upper [Obsolete Name And Code: Use CFML] (UGL) Upper Grey Limestone [Obsolete Name and Code: Use CFML] (-2684) D2 Limestones [Obsolete Name and Code: Use CFML] (-1421) |
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| Alternative Name(s): | none recorded or not applicable | ||
| Stratotypes: | |||
| Partial Type Section | Cefn Mawr Quarry. Exposes the conformable contact with the underlying pale massive limestones of the underlying Loggerheads Limestone Formation, and the lower 80m of the Cefn Mawr Limestone (Davies et al, in press) | ||
| Reference(s): | |||
| Davies, J R, Wilson, D and Williamson, I T. 2004. Geology of the country around Flint. Memoir for 1:50 000 Geological Sheet 108. | |||
| Somerville, I D, 1979. Cyclotherms in the early Brigantian (lower D2) limestones east of the Clwydian Range, North Wales. Geological Journal, Vol.14, 69-87. | |||
| Morton, G H 1876-8. The Carboniferous Limestone and Millstone Grit of North Wales. Proceedings of the Liverpool Geological Society. Vol.3, 190-201. | |||
| Wedd, C B, Smith, B and Wills, L J. 1927. The geology of the country around Wrexham, Part 1. Lower Palaeozoic and Lower Carboniferous rocks. Memoir of the Geological Survey of Great Britain. [London: HMSO.] 179pp. | |||
| Warren, P T, Price, D, Nutt, M J C and Smith, E G. 1984. Geology of the country around Rhyl and Denbigh. Memoir for 1:50 000 geological sheets 95 and 107 and parts of sheets 94 and 106. | |||
| Waters, C N, Waters, R A, Barclay, W J, and Davies, J R. 2009. Lithostratigraphical framework for Carboniferous successions of Southern Great Britain (Onshore). British Geological Survey Research Report, RR/09/01. 184pp. | |||
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: | |||
| E108 E121 E137 E096 E093 E094 E120 | |||